There has been provided a laser printer for forming color images. For example, a laser printer employing a direct-transfer method includes a plurality of photosensitive members for respective colors, and a developing unit having a plurality of developing rollers for forming toner images with toner of each color corresponding to respective electrostatic latent images on the photosensitive members. The toner images are transferred from the photosensitive members onto a printing sheet of paper conveyed on a conveying belt to form a color image.
Some type of laser printer has a color mode for forming color images and a monochrome mode for forming monochrome images only with black toner. In the monochrome mode, only a developing roller for black is placed in contact with a corresponding photosensitive member, and remaining developing rollers are kept out of contact with corresponding photosensitive members. In the color mode, on the other hand, all of the developing rollers are in contact with the corresponding photosensitive members.
Some of the toner constituting the toner image on each photosensitive member may not be transferred onto a printing sheet and remains on the photosensitive member. Also, toner of opposite polarity clinging on the conveying belt may be transferred onto the photosensitive member when transfer operation is performed. In order to remove such toner (residue toner) and other excrescences from the photosensitive member, a collecting roller may be provided for each photosensitive member.
Specifically, a predetermined bias is applied to each collecting roller to transfer the excrescences from the photosensitive member onto the collecting roller. Then, the excrescences collected onto the collecting roller are transferred back onto the photosensitive member and further onto the conveying belt. This operation is referred to as “reverse-transfer operation” hereinafter. Thereafter, the excrescences are removed from the conveying belt with a cleaning member disposed in contact with the conveying belt.